The electrodeposition of copper is used extensively in a variety of industrial processes including electroplating and electroforming. The many and varied uses for electrodeposited copper generally require somewhat different combinations of physical properties, such as electrical conductivity, hardness, ductility, film forming properties, adhesion to other materials, brightness and the like. In addition to having the specific properties noted above, electrodeposited copper, in general, should likewise be relatively easy to machine into precise configurations.
Special electrodeposition baths have been developed for the electrodeposition of copper used for different types of applications. The typical electrodeposition baths are primarily comprised of aqueous acidic solutions of copper sulfate. Selected additives are typically added to the electrodeposition bath to vary properties of the copper for a given application. Usually a combination of additives is employed, with each additive being included in the electrodeposition bath for a particular purpose. Conventional additives include materials such as thiourea, dextrin, molasses and the like to control the electrodeposition process. Aliphatic and aromatic quaternary amines can be added to improve the brightness of the electrodeposited copper. Polysulfides are also commonly used as additives to improve the ductility of the electrodeposited copper.
More recently, dyes of the phenazine class and, more specifically, the phenazine azo dyes, have been extensively employed to improve the leveling properties of electrodeposition baths and to improve the brightness and mechanical properties, such as ductility, of the electrodeposited copper.
The members of the phenazine class of dyestuffs which have proven to be most useful are represented by the formula ##STR1## wherein R.sub.1 to R.sub.4 are the same or different and can be hydrogen, methyl, or ethyl and R.sub.5 can be hydrogen, --NH.sub.2, --N(CH.sub.3).sub.2 and --N.dbd.N--Z where Z is an aromatic coupling group selected from the group consisting of phenyl, naphthyl or phenyl and naphthyl radicals substituted with amino, alkyl, substituted alkyl, hydroxy and alkoxy substituents, and X is an anion selected from the group consisting of Cl.sup.-, Br.sup.-, SO.sub.4.sup.-- and HSO.sub.4.sup.-.
Among the above dyestuffs of the phenazine class, the phenazine azo dyestuffs are the most effective members of the class to improve the brightness and other related properties of electrodeposited copper, in particular, the Janus Green B type dyestuffs.
The phenazine class of additives and, in particular, those additives comprised of Janus Green B type dyestuffs, have wide commercial acceptance for use in the electrodeposition of copper for conventional applications. The resulting deposited copper is bright and can readily be machined and finished using conventional machining tools and methods within generally acceptable tolerances required for relatively large scale articles.
Recently, however, there have been a number of applications which require copper substrates and the like which must be precisely machined with features which are 10 to 100 times smaller in size than the features obtainable using conventional machine tools and machining techniques. These new applications include, for example, recording substrates for high density information discs, such as optical discs, compact audio discs, capacitance electronic discs and the like, which typically have information tracks which are 0.5 micrometer in depth with information tracks being spaced at about 4000 tracks per centimeter. In order to be used for micromachining, a copper substrate must have proper grain size and hardness. Satisfactory electrodeposited copper for micromachining has a grain size from about 200 to about 500 Angstroms and a hardness of from about 250 to about 320 on the Knoop hardness scale measured at 15 grams. In addition, copper substrates having the above-noted properties and which are also bright and have fine laminar structures exhibit excellent micromachinability. The ultimate test, however, is that the electrodeposited copper must exhibit excellent diamond turnability when micromachined.
Another problem that has become apparent is that the electrodeposition baths have batch to batch variations which adversely affect the deposited copper properties. Micromachining problems are encountered with electrodeposited copper made with different additives and using the same additives under apparently identical operating conditions. As a result of trial and error, it has been found the electrodeposition bath which contained, as an additive, a Janus Green B type dyestuff generally provides a higher proportion of satisfactory copper parts for micromachining. However, even using the Janus Green B type dyestuff as an electrodeposition additive, the electrodeposited copper which is obtained varies widely in properties from batch to batch and even with electrodeposited copper parts made in the same electrodeposition bath but at different times in the production cycle.
In order to determine if impurities were causing the problems, chemically pure samples of Janus Green B type dyestuffs were evaluated as additives, but unexpectedly even more erratic and unsatisfactory results were obtained. Mixtures of different Janus Green B type dyestuffs were also evaluated with no better success than that obtained with a single type of Janus Green B type dyestuff.
The problems that were encountered were, among other things, that the electrodeposited copper was either too soft or too hard for micromachining. In the micromachining process, electrodeposited copper that is too hard will not machine properly and expensive, delicate tools are often broken in the micromachining process. Electrodeposited copper which is too soft micromachines unevenly and tends to be easily gouged and burrs frequently are formed on the machined surfaces.
What would be highly desirable would be an electrodeposition bath additive, an electrodeposition bath composition, and a process for electrodepositing copper which would consistently result in micromachinable electrodeposited copper.